It probably warns people to flee from the metropolitan areas, because those areas face imminent destruction from the American Air Force. Conventional bombing was a tremendous threat too (more people were killed in one night by the conventional firebombing of Tokyo than by either atomic bomb drop), and it was a standard tactic to try to scare the urban population into abandoning their homes and fleeing to rural areas...thus abandoning their jobs in Japanese war industries and the general infrastructure, thus lessening Japan's ability to fight the war. That was the intent. I doubt that there was much real concern about the lives of Japanese civilians, but if you could get them to abandon the cities, then you could basically destroy the Japanese economy. The same tactics were applied in Germany, but (fortunately) without resorting to atomic weapons. Cities were still destroyed quite effectively, in any case, by means of ordinary conventional bombing.